In the following description, reference is made purely by way of example to the packing industry, and in particular to cigarette packing machines for producing rigid hinged-lid packets of cigarettes, and on which a finished packet is normally formed by folding a preweakened blank of cardboard about a respective group of cigarettes wrapped in a sheet of foil material.
In the cigarette packing industry, it is common practice to us step-operated packing machines on which a blank is normally folded about a respective group of cigarettes as the group of cigarettes is transferred between the seats of successive packing wheels. In particular, a first folding operation, during which the blank assumes a substantially U-shaped configuration about the group of cigarettes, is normally performed by transferring the group of cigarettes from one seat on one wheel to a corresponding seat on a follow-up wheel, and by interposing the blank between the group and the seat on the follow-up wheel.
On step-operated packing machines, folding blanks about respective groups by transferring the groups between successive wheels is, mechanically speaking, fairly straightforward, seeing as the two corresponding seats of the two successive wheels are kept facing each other for a given time interval between successive operating steps of the wheels. Functionally speaking, however, folding the blanks as described above involves serious drawbacks on account of the ever-increasing operating speed of modern step-operated packing machines continually reducing the time interval and so increasing the likelihood of the blanks being damaged or folded incorrectly.
On continuous packing machines, folding the blanks by transferring the groups between successive wheels poses fewer operating problems, but seriously complicates the structure of the machine by requiring pairs of packing wheels comprising movable seats, and wherein each seat on each wheel is related dynamically to a corresponding seat on another wheel. That is, for a folding operation to be performed, two corresponding seats--one on one wheel and the other on a follow-up wheel--must be kept facing each other for a given length of time, while moving at least one of the two corresponding seats with respect to the relative wheel.